1. A nonlinear analog circuit or device that accepts as its input two different frequencies (signals) and presents at its output (a) a signal equal in frequency to the sum of the frequencies of the input signals, (b) a signal equal in frequency to the difference between the frequencies of the input signals, and (c) if they are not filtered out, (c) the original input frequencies. Note: A nonlinear mixer may be used for amplitude modulation (AM) of rf carriers. See heterodyne. 2. A linear analog circuit or device that accepts as its input two or more different signals (e.g., from audio channels), adds them algebraically, and presents the sum at its output. Note: For example, a linear mixer of audio signals may be used to combine signals from several microphones for purposes of recording on a common audio track.